Microsomes isolated from whole rat brain were found to contain both cytochrome P-450 (0.025-0.051 nmoles/milligram) and NADPH cytochrome c reductase activity (26-55 nmoles/milligram/min). The brain cytochrome P-450 apparently catalayzes the oxidation of estradiol to a reactive metabolite that became covalently bound to rat brain microsomal protein. The covalent binding of estradiol was inhibited 63% by an atmosphere of CO:O2. Moreover, an antibody prepared against NADPH cytochrome c reductase inhibited not only the covalent binding of estradiol but also the covalent binding of 2-hydroxyestradiol and catecholamines by rat brain microsomes.